FIGS. 4 to 6 are longitudinal cross-sectional views of a conventional traveling wave tube. Specifically, FIG. 4 shows a conduction cooling type, FIG. 5 a forced air-cooling type and FIG. 6 shows a water-cooling type.
FIG. 7 shows a longitudinal cross-sectional view of a conventional collector, while FIG. 8a is a transverse cross-sectional view of another illustrative structure of a conventional collector and FIG. 8b is a longitudinal cross-sectional view of FIG. 8a. FIG. 9 shows a two-stage version of the collector shown in FIG. 8. Specifically, FIGS. 9a and 9b are a transverse cross-sectional view and a longitudinal cross-sectional view of the collector, respectively.
Among the devices which effect amplification of the micro-wave, using an electron beam, there is a traveling wave tube used as a relay station for micro-waves and for satellite communication.
Referring to FIGS. 4 to 6, a traveling wave tube includes an electron gun 23 for radiating an electron beam 24, a delay wave circuit 25 for producing interaction between the electron beam 24 and the input micro-wave, a collector 26 for collecting the electron beam 24 and a beam converging device 27 for converging the electron beam 24.
The electron beam 24 emitted by the electron gun 23 traverses the delay wave circuit 25 to amplify a signal and is collected by the collector 26. At this time, the electron beam 24 captured by the collector 26 has its kinetic energy converted into thermal energy to raise the temperature of a collector electrode 28.
For this reason, the heat generated in the collector electrode 28 needs to be externally released. Among the methods for releasing the heat, there are a conduction cooling type collector 31 for releasing the heat from a base plate 30 to a heat sink 29, as shown in FIG. 4, a forced air-cooling type collector 33 by providing a fin 32 on the outer periphery of the collector for flowing air thereon to release the heat with assistance from base plate 30, as shown in FIG. 5, and a water-cooling type collector 35 by providing a water-cooled pipe 34 passed through by water to release the collector heat with assistance from base plate 30, as shown in FIG. 6.
For increasing the efficiency of the traveling wave tube, a method known as collector potential lowering method is used. This method consists in progressively lowering the potential applied across the collector electrode 28 (see FIG. 4) relative to the delay wave circuit 25 to lower the speed of the electron beam 24 colliding against the collector to decrease the energy generated in the collector electrode 28. To this end, there is provided a collector-insulating ceramic element 36 for maintaining insulation between the collector 26 and the delay wave circuit 25 against high voltage.
Referring to FIG. 7, the collector structure of a conventional traveling wave tube includes an insulating enclosure 37 of ceramics etc. for insulating the collector electrode 28, formed of copper, molybdenum or graphite etc., for maintaining vacuum and air-tightness, a base plate 30 for supporting the collector electrode 28 and passing heat generated in the collector electrode 28 through the insulating member to release heat to the outside and a collector-insulating ceramic element 36 for maintaining insulation against the delay wave circuit. These component parts are usually connected together by brazing or welding etc. This structure is termed an external insulation type collector 38.
In the above-described collector, there are occasions where the collector electrode is displaced in the axial direction due to mechanical vibrations or impact. This positional deviation occasionally leads to changes in the colliding position of the electron beam leading to emission of gases of or leading to an increased amount of retrogressive electrons to increase the helical current of the traveling wave tube, occasionally leading to destruction of the tube bulb.
Also, in such a collector structure, a problem arises in that RF components (TEM mode) of the electron beam incident on the collector are subjected to RF leakage through a collector lead line or collector-insulating ceramic element.
In JP Patent Kokai JP-A-2-101454(1990), there is shown a structure in which the collector electrode 28 is supported by plural heat-conductive columnar ceramic elements 40 arranged between the collector electrode 28 and the external enclosure 41 to improve vibration-resistance and resistance against impact, while maintaining voltage withstand characteristics, as shown in FIGS. 8a and 8b. This structure is termed an internal insulation type collector 39.
In this type of collector, the outer enclosure 41, collector electrode 28 and the highly heat-conductive columnar ceramic elements 40 are secured in position by deforming the outer enclosure 41 such as by press-working. If, in this structure, plural collector electrodes 28, specifically a first collector electrode 42 and a second collector electrode 43, are used and arranged in this order from the upstream to the downstream side of the electron beam, it is possible to lower the potential of the second collector electrode 43 relative to the first collector electrode 42 to improve the overall efficiency of the traveling wave tube.
A two-stage collector 44 is illustrated in FIGS. 9(a) and 9(b). An electron beam emitted by the electron gun is passed through the delay wave circuit to amplify the signal and is captured by the first collector electrode 42 and the second collector electrode 43 which are supported by heat-conductive columnar ceramic elements 40.
For improving the efficiency of the traveling wave tube, there is usually employed a method of lowering the potential of the first collector electrode 42 to approximately 50% of that of the delay wave circuit and of lowering the potential of the second collector electrode 43 to approximately one-half that of the first collector electrode 42. This method consists in sequentially lowering the collector potential with respect to the potential of the delay wave circuit to lower the speed of the electron beam impinging against the collector in order to lower the energy produced in each collector electrode while improving the overall efficiency of the traveling wave tube.
Referring to FIG. 9(b), in the case of a two-stage internal insulation type collector 44, it is necessary to take out a first collector lead wire 45 of the first collector electrode 42 to a position forwardly or rearwardly of the outer enclosure 41. In the present case, the collector lead wire is taken out to a position rearward of the outer enclosure 41 and is passed through the inside of an insulator tube 46 for maintaining insulation of the first collector lead wire 45 to reduce the size of the collector as shown in FIG. 9(b). This insulator tube 46 is led rearwardly of the collector via a groove 47 (see FIG. 9(a)) formed in a portion of the outer periphery of the second collector electrode 43 so as to be led to outside vacuum in an insulated state from a second collector lead line 48 (see FIG. 9(b)).
In this structure, the electrical field is unavoidably concentrated in an edge portion of the insulator tube 46. Since there is a corner portion of the groove 47 in this edge portion, a problem arises in that the withstand voltage is deteriorated between the first collector lead line and the ground potential.
In addition to this structure, a demand is raised in recent years for providing a small-sized lightweight structure easy to manufacture for communication or loading on a satellite to lower the cost.